With the development of national economy, energy consumption increases rapidly. A large quantity of flue gases is produced by combustion of lots of fossil fuels and discharged into atmosphere, wherein besides sulfur dioxide, sulfur trioxide, hydrogen chloride, hydrogen fluoride, nitrogen oxides and a small quantity of harmful organic substances, a large quantity of dusts is contained, and there are tiny hydrophilic and non-hydrophilic particles in these dusts (mainly calcium salt particles, aluminum salt particles, magnesium salt particles, titanium salt particles, iron salt particles, lead salt particles, zinc salt particles, cobalt salt particles, rare earth element particles, radioactive element particles and particles of other harmful elements, as well as mineral particles such as silica particles, mullite particles, silicate particles, and phosphate particles). These particles are discharged together with the flue gases into atmosphere, and at the same time, sulfur dioxide, sulfur trioxide, hydrogen chloride, hydrogen fluoride, nitrogen oxides, harmful organic substances, bacteria, and the like are readily adsorbed on the surface of these particles, thus the content of atmospheric suspended particles (which are generally referred to as PM100, PM10, PM2.5, etc.) is increased significantly, resulting in the phenomena of haze and atmospheric photochemical reactions, and causing more and more serious environmental pollution. The atmospheric smoke, acid rain, greenhouse effect and the destruction of ozone layer have become four killers endangering human survival.
Among them, harmful substances such as smoke, sulfur dioxide, nitrogen oxides or the like are the main reasons for air pollution, acid rain and greenhouse effect. In recent years, the management of nitrogen oxides (NOX, including a variety of compounds, such as N2O, NO, NO2, N2O3, N2O, N2O5 or the like) has become one of the focuses of attention by people. Under conditions of high temperature combustion, NOX is mainly present in the form of NO, and NO constitutes about 95% of the initially discharged NOX. However, atmospheric NO reacts very easily with oxygen in the air to produce NOX, thus atmospheric NOX is generally present in the form of NO. In the air, NO and NO2 undergo mutual conversion by photochemical reactions to achieve equilibrium. At higher temperatures or in the presence of clouds and mists, NO2 further interacts with water molecules to form the second important acid component in the acid rain, that is nitric acid (HNO3), and in the presence of a catalyst, such as under appropriate meteorological conditions, the conversion of NO2 to nitric acid is accelerated. Especially when NO2 and SO2 are present simultaneously, they can catalyze each other, and the formation of nitric acid is even faster. In addition, NOX may also accumulate gradually due to the waste gases discharged into stratosphere by aircrafts, such that its concentration increases, then at this point NO reacts with O3 within the stratosphere to produce NO2 and O2, and NO2 further reacts with O2 to produce NO and O2, thus O3 equilibrium is broken, and the concentration of O3 is reduced, resulting in the loss of O3 layer. Research data shows that, if the management of nitrogen oxides in flue gases is still not enhanced, both the total amount of nitrogen oxides and the proportion thereof in the atmospheric pollutants will rise, and nitrogen oxides may replace sulfur dioxide as the main pollutants in atmosphere.
China is one of the few countries in the world that takes coal as the main source of energy. According to statistics, 67% of China's nitrogen oxides (NOX) emissions are from coal combustion. According to statistics of State Environmental Protection Administration, in 2005 and 2010, coal consumption in China's thermal power plants accounted for 56% and 64% respectively of the total coal consumption in China, and NOX production by thermal power plants accounted for 50% of the total production in China. In view of the contribution of coal consumption to NOX emissions, the control of NOX emissions in thermal power plants is a key factor to control the total NOX emissions in China. With the promulgation and implementation of China's latest “Air Pollutants Emission Control Standards for Thermal Power Plants” and “Air Pollution Prevention and Control Law” as well as the formal entry into force of “Kyoto Protocol”, the control of NOX emissions in China will become increasingly stringent, and it is imperative to take effective measures for controlling NOX emissions in thermal power plants. At present, the desulfurization and denitration processes of flue gases are independent of each other, denitration is usually carried out first and followed by desulfurization.
Now the denitration process used in the actual production mainly contains selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR).
In selective catalytic reduction (SCR), a catalytic bed or system is utilized to process a flue gas stream, wherein ammonia or urea is injected into the flue gas and mixed, then the gas is passed through the catalyst layer, and NOX is selectively converted (reduced) to N2 and H2O. SCR method is currently the most proven denitration technology with highest denitration efficiency. The first demonstration project of the SCR system was established in Shimoneski power plant in Japan in 1975, afterwards the SCR technology was widely applied in Japan. In Europe, there have been successful application experiences from more than 120 large-scale devices, and the NOX removal rate can reach 80% to 90%. So far, there are approximately 170 sets of device in Japan, power plants with a capacity of close to 100 GW have installed such apparatus, and US government also uses the SCR technology as the main technology for the main power plants to control NOX. It is reported that the SCR method has currently become a relatively proven mainstream technology for denitration in power plants at home and abroad.
The principle of flue gas denitration by SCR method is as follows: under the catalysis of catalyst with TiO2 and V2O5 as the main components and at a temperature of 280 to 400° C., or under the catalysis of catalyst with TiO2, V2O5 and MnO as the main components and at a temperature of higher than 180° C., ammonia is sprayed into the flue gas, and NO and NO2 are reduced to N2 and H2O, to achieve the purpose of denitration.
The SNCR denitration technology is a selective non-catalytic reduction technology without the use of catalysts, wherein at a temperature in the range of 850 to 1100° C., an amino-containing reducing agent (such as aqueous ammonia, urea solution, etc.) is sprayed into a furnace, and NO and NO2 in the flue gas are reduced to N2 and H2O, thus the purpose of denitration is achieved. However, NOX removal rate of the industrial SNCR system is only 30-70%.
Both in SCR and SNCR denitration processes, ammonia consumption is relatively large, as the flue gas contains about 4% to 9% O2, ammonia gas or amino-containing urea will react with O2 to produce NOX, ammonia is thus consumed, meanwhile ammonia reacts incompletely, some ammonia is discharged into atmosphere together with the flue gas, and the loss of ammonia increases, resulting in the phenomenon of secondary pollution. Lots of fossil fuels are consumed during ammonia production, and a large quantity of waste gases, waste dregs and waste water is produced, which is a severe process of environmental pollution, thus the use of ammonia should be avoided as far as possible.
There are also some drawbacks in the existing methods for removing NO from flue gases by SCR and SNCR. For the NO removal methods with ammonia as the reducing agent, ammonia, urea or aqueous urea solution is generally used as the source of the reducing agent. Excessive injection of ammonia or urea will lead to the so-called ammonia penetration, and the discharged ammonia is even more harmful than the discharged NOX. The oxidation of excessive ammonia may lead to the formation of NOX, and the transportation and storage of ammonia reducing agent have high requirements for safety and environmental protection. In addition, the catalyst used in the process of denitration will suffer from impingement and abrasion by high-concentration smoke and contamination by impurities in fly ashes. Excessively high temperature of flue gas will lead to catalyst sintering and deactivation, and the presence of SO2 will lead to a rapid decline in catalyst activity.
Many researchers at home and abroad propose to use ozone for simultaneously oxidizing SO2 and NO in a flue gas to SO3 and NO2, and then lime/limestone, sodium hydroxide, etc. are used for absorption, thus achieving the effect of simultaneous removal of SO2 and NO. However, since ozone-generating device is very expensive, a great investment is required; and ozone production cost is very high, that for the oxidation of 1 mole of SO2 to SO3 or of 1 mole of NO to NO2, the ozone consumption required is 1.5 to 3 moles, respectively, while for producing 1 kg of ozone, about 10 kWh of electricity and 10 to 20 kg of pure oxygen are to be consumed, respectively; the energy consumption is large, the expenditure is high, and the investment is great, making the large-scale industrialization of flue gas desulfurization and denitration by ozone unachievable currently. CN101352645A discloses a denitration process by catalytic oxidation, wherein the catalyst uses TiO2 or ZrO2—TiO2 as the carrier and Co as the active component. NO is oxidized to water-soluble NO2 by the oxygen contained in the flue gas itself, and then an alkaline solution is used for absorption and nitrogen oxides are thus removed.
CN1768902A discloses a boiler flue gas denitration method, wherein ozone O3 is sprayed into a low-temperature section in a temperature range of 110-150° C. of the boiler flue, and nitric oxide NO in the boiler flue gas is oxidized to water-soluble nitrogen oxides of high valences, such as NO2, NO3 or N2O5; the molar ratio of the sprayed ozone O3 to NO in the boiler flue gas is 0.5 to 1.5, and then the nitrogen oxides in the flue gas are removed by washing with an alkaline aqueous solution. However, in actual use, this technology has relatively low denitration efficiency and very high ozone consumption. To meet emission standards, its operating cost is particularly high, and enterprises cannot afford it, so large-scale industrialization of this technology has always been unachievable.
The patent application no. 201410245417.4 discloses a process and an apparatus for simultaneous desulfurization and denitration of a flue gas, wherein SO2 and NO in the flue gas can be removed at the same time. However, upon further research and evaluation, it is found that, although the desulfurization efficiency of the process is greater than 99% and the content of SO2 in the flue gas can be reduced to below 30 mg/Nm3, yet the denitration efficiency is as low as about 40%-80%, and the regenerated gas released by regeneration is a mixed gas composed of CO2, SO2 and NO, later separation and disposal of which are relatively difficult due to the process complexity, waste recycling is troublesome, and it is inconvenient to turn wastes into valuables.
There are still a variety of problems in the above denitration embodiments, and there has been no denitration method in prior art which can effectively remove NO from waste gases and be used for industrial production.